The World Health Organization recommends that all HIV-exposed infants begin co-trimoxazole preventive therapy (CPT) between four and six weeks of age, and continue CPT until at least six weeks after cessation of breastfeeding and definitive exclusion of HIV infection. As early infant HIV diagnosis services are becoming more widely available in Africa, and as more efficacious drug regimens for the prevention of mother-to-child HIV transmission (PMTCT) are being adopted and extended through the breastfeeding period, the population of HIV-exposed, uninfected infants is growing. While the case for universal provision of CPT to HIV-infected infants is strong, little is known about the impact of universal provision of CPT to HIV-exposed, uninfected infants. As co-trimoxazole is an antimalarial agent, it will be imperative to know the impact of daily CPT on the prevention of malaria among HIV-exposed but uninfected infants, and whether its use leads to decreased parasite density when malaria episodes do occur. The Breastfeeding, Antiretroviral and Nutrition (BAN) study (ClinicalTrials.gov number, NCT00164736) provides an optimal study population for this research. A total of 259 HIV-exposed, uninfected infants enrolled between 2004 and 2006 met eligibility criteria and did not receive CPT due to compliance with the Malawian national guidelines. Beginning in 2006 Malawi revised their CPT policy resulting in 626 HIV-exposed, uninfected infants enrolled in BAN being initiated on CPT starting at 4 to 6 weeks of age who met eligibility criteria. The goal of this project is to 1) determine the effect of CPT on malaria incidence and 2) determine the effect of CPT on mean parasitemia levels. For Aim 1, malaria will be defined as the combination of clinical malaria (clinical symptoms with documented microscopy positive parasitemia) and PCR positivity on blood spots to determine asymptomatic parasitemias taken at 12, 24, and 36 weeks of age. For Aim 2, we will combine existing microscopic parasitemia data with parasitemias determined by a novel quantitative real time PCR assay from the blood spots. A binomial regression and linear regression model using generalized estimating equations will be used to estimate a risk ratio for malaria by CPT status, and mean difference in malaria parasitemia by CPT status respectively. This research will allow us to compare incidence of malaria (Aim 1) and mean difference in malaria parasitemia (Aim 2) during the first 36 weeks of life among HIV-exposed, uninfected infants that did and did not receive daily co-trimoxazole beginning at 6 weeks of age. As the PMTCT and pediatric HIV landscape continues to change, it is imperative to critically and scientifically assess the benefit of universal provision of co-trimoxazole preventive therapy to HIV-exposed infants. This research will add to the limited body of evidence on CPT in HIV-exposed uninfected infants, and will have direct implications for Malawi's national well-child, malaria, and HIV programs.